The bug

Unsupported sand/gravel two or more deep, doesn't react to blocks placed on it.

What I expected to happen

The sand to collapse downwards to a solid floor.

What actually happened

The upper sand in the pile basically acted like it was on a solid block.

How to reproduce

1. Create a new world eg: -8425874089412349429
2. Find an area with unsupported Sand
       131 69 -92 = 2 unsupported
       130 69 -92 = 3 unsupported
       129 69 -92 = 4 unsupported
3. Place a block above each of them and nothing happens

Note

The same thing happens when a collapse of 1 high or more (from the bottom) signals the upper sand of a 2+ unsupported stack alongside it to collapse, that 2+ doesn't, it seems that the supported check only applies to that block and travels sideways and upwards, but NOT downwards.

Optional suggestion

Rather than have these wait to react to a player, consider collapsing them once a player is 1 chunk away, that way they are pre-collapsed without having to wait for a player to trigger them, accidentally or otherwise, without needing to collapse every floating collapsible block occurrence in that world.